1. Field of the Invention
The present invention relates to a radio telephone in which receiving and transmitting operations are carried out using a spread spectrum communication system in accordance with a time division duplexing. More particularly, the invention relates to a call processing method in a radio telephone capable of sending a ring signal from the base set of the radio telephone to all handsets when the base set receives an incoming call, and then establishing a communication channel between the base set and the handset transmitting a signal responding to the ring signal.
2. Description of the Related Art
The spread spectrum communication system is the system which has been long used for military communications. This system has a high resistance to jamming or interference and a superior privacy function. In such a spread spectrum system, information is transmitted while its bandwidth is spread out using a pseudo noise (PN) code with a bandwidth considerably wider than that of the information. For example, voice signals generally occupy a bandwidth less than 20 kHz wide. In a spread spectrum system, signals representing the voice information may be spread out over a bandwidth spanning several MHz or more. In receiving, information is decoded by using the same PN code as that used in the transmission. In this way, reception of only the desired information is achieved.
One type of diffused band system is known as code division multiple access (CDMA), in which different PN codes are respectively allocated to different users. As a result, the users can be distinguished from one another, such that multiple users can be simultaneously connected using the same frequency band. Recently, CDMA has been advanced to digital mobile communication systems of cellular telephone networks.
A CDMA technique may be used in operating a radio telephone consisting of a base set and a plurality of handsets radio linked to the base set. For example, this type of radio telephone may be used in the home or office, where the base set is connected to a telephone line of a wire-line telephone network. In order to apply the CDMA technique to such a multi-handset operation, it is necessary to allocate a different PN code to each handset thereby distinguishing the handsets from one another. The base set may be equipped with an equal number of transmitting units and receiving units, both corresponding in number to the handsets. Since the receiving units include synchronization circuits, their configurations are considerably more complex than the transmitting units. For this reason, the configuration requiring as many receiving units as there are handsets, is undesirable.
Meanwhile, the base set may be connected to the handsets via only one channel because there is only one central office line connected to a switching system in most radio telephones. In other words, the base set may communicate with all handsets in a one-to-one communication fashion using only a single transmitting unit while performing a change in PN code in accordance with the handsets. However, where an externally generated incoming call is received, it is necessary for the base set to send a ring signal to all handsets.